Application Of Fiber Grating Sensing Technology In Strain Gradient Measurement

Strain gradient monitoring has the advantage of more sensitive and earlier capture of damage in structural health monitoring.In order to meet the long-term health monitoring needs of large-scale civil engineering structures,the plane strain gradient theory was analyzed,and a fiber grating strain gradient test equipment was developed.After three-point bending beam strain gradient test experiments,the device was tested to detect the strain gradient of the structure.The main research contents of this paper include the following aspects:Applying the high-order Cauchy-Born criterion,the formula of the coordinate transformation of the plane vector under the old and new coordinate systems is established,and the relational expression of the strain gradient in the new coordinate system is deduced.Based on the analysis method of plane strain state,applying the chain rule and the transformation relation of strain gradient in the new coordinate system,the extreme conditions of strain gradient are analyzed and solved.By comparing the calculated extremum size,the maximum value of the strain gradient and its direction are determined,which provides a theoretical basis for the layout of the sensor on the surface of the component.In order to measure the strain gradient of component deformation,a set of fiber grating strain gradiometer sensing equipment was developed.The core parts of the equipment are fiber grating strain gradient analyzer and fiber grating strain gradient analysis software.The fiber grating strain gradient analyzer has higher wavelength identification accuracy and can output high resolution fiber grating reflection spectrum maps.Fiber grating strain gradient analysis software identifies the center wavelength,3dB bandwidth and other information through a Gaussian fitting algorithm.The linear relationship between the amount of bandwidth expansion and the strain gradient and the plane strain gradient theory are used to solve the strain gradient of the component.By using the gradient strain generated by the pressure of the three-point bending beam,the Bragg reflection wavelength shift is achieved while the grating is driven to broaden the bandwidth,thereby realizing the strain gradient sensing.Based on the theory of strain gradient derived from the material mechanics,the strain gradient measurement and theoretical values of the fiber grating strain gradient sensing equipment were compared to verify the feasibility and applicability of the strain gradient measurement.The accuracy of the measurement results depends on the coupling problem of the FBG sensor and the component.In the case of a good coupling between the FBG sensor and the object to be measured,the strain gradient test works well.